sedimentary

Dendritic Siltstone (or Mudstone)

Siltstone with Manganese Oxide dendrites

Hardness: 3-4 on Mohs scale; Color: Tan to yellowish-brown matrix with black arborescent (tree-like) markings; Luster: Dull/Earthy; Crystal structure: Microcrystalline/Clastic; Cleavage: None (conchoidal or irregular fracture); Specific gravity: 2.1-2.6

Hardness: 3-4 on Mohs scale
Luster: Dull/Earthy

Identified March 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the lithification of silt-sized particles in low-energy aquatic environments. The black 'fern' patterns are manganese oxides that precipitated from mineral-rich groundwater filling micro-fractures in the rock over thousands of years.

Uses & applications

Primarily used as decorative stone, for garden landscaping, or as inexpensive educational rock specimens. High-quality slabs are occasionally used for cabinet making or jewelry inlays.

Geological facts

The black markings are frequently mistaken for plant fossils (pseudofossils) because they resemble moss or ferns. They are actually a fractal growth pattern of mineral crystals.

Field identification & locations

Identify in the field by looking for flat, earthy-textured stones in sedimentary basins that feature black, branching patterns. Can be found globally in areas with shale or siltstone deposits, such as the American Southwest.

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